This invention relates generally to digital data transmision techniques wherein dial-up telephone circuits are utilized as the transmission media, and more particularly to an improved modem for accomplishing such transmission.
Transmission of digital data between distant locations by use of dial-up telephone circuits is becoming increasingly popular. The telephone system is used because of the convenience of an existing system interconnecting locations between which data is desired to be transmitted. Because the main purpose of the telephone system is to transmit only voice communications, telephone circuits are very limited in their ability to transmit data. Accordingly, there has been considerable attention directed to making the maximum use of available telephone circuits for data transmission. A principle limitation of telephone circuits for data transmission is their narrow bandwidth, essentially within a range of about 100 Hz to 3200 Hz. Modems have been developed for interconnecting digital equipment to the telephone circuits in a manner to utilize these circuits. Since the rate of digital data that can be transmitted depends upon the telephone circuit bandwidth that can be utilized, modem development has been directed largely toward making better use of the available circuit bandwidths.
The use of common two-wire telephone circuits which transmit signals in both directions at the same time creates problems in full duplex data transmission. Echos of data transmitted at one end of a circuit generally occur and return to be received with the desired data signals at that end. Two components of such undesirable echos are recognized: First, there is an echo from a remote location that is significantly attenuated by the telephone circuit but which is also highly unpredictable because of the time-varying characteristics of trunk telephone carrier circuits. The second is a local echo which occurs in the portion of the telephone circuit at the transmitting location, a local echo being very strong in signal level but generally easier to predict because it has been processed by only linear, time-invariant elements. The goal in dealing with these echos is to reduce the data errors caused by them to a tolerable level. The problem is eliminated if separate telephone circuits (that is, four-wire leased line circuits) are used for independently transmitting data in each direction between two locations, but it is however generally more desirable to be able to use a single dial-up telephone circuit of the type installed anywhere, and so these echo problems must be dealt with. The transmission of data over a single two-wire telephone circuit in only one direction at a time (half duplex operation) also eliminates these problems, but it is generally desired to be able to communicate data simultaneously in both directions (full duplex operation).
A common technique with existing telephone modems is to utilize separate portions of the telephone circuit bandwidth for transmission and reception. In order to avoid the echo problems, these two bandwidths are significantly separated by filters in order to be independent, thus not using a significant portion of the available telephone circuit bandwidth. This results in a significant limitation in the data rate that can be communicated because of the limited bandwidth of the send and receive channels.
The technical literature alternatively suggests utilizing the full bandwidth of the telephone circuit for simultaneous transmission of data in both directions. The undesirable echos that will cause interference between the two channels are suggested to be eliminated by predicting at each modem the echo that will be received by it as a result of the data transmitted from that modem, the predicted echo signal then being subtracted from the received signal to eliminate the effects of the echo. The echo cancellation technique is generally satisfactory to eliminate effects of the local echo, but remote echos are highly unpredictable and extremely difficult to eliminate because they have passed through time-varying elements in the telephone carrier system. As a result, this suggested technique has not been found to be employed in any practical equipment capable of transmitting data with an acceptably low error rate.
Therefore, it is a primary object of the present invention to provide an improved technique of transmitting digital data in full duplex via dial-up telephone circuits with a high speed by an efficient use of the available telephone circuit bandwidth but also with an acceptable low error rate.
It is another object of the present invention to provide a telephone modem of simple construction for accomplishing such data transmission.